The importance of networks or network services has risen steadily in recent years. In addition to their generally known use as a communication platform, e.g., the Internet, the use of networks in industrial environments is also growing in importance, e.g., in networked control and automation systems.
In industrial applications, in particular, an error-free, continuously available network connection between the individual stations is important to avoid production problems or even outages.
However, a connection between the stations which remains uninterrupted over time can never be guaranteed, since problems may always occur within the connection, e.g., cable ruptures and the like.
In network technology, therefore, a number of methods exists for detecting connection problems and eliminating them, if necessary.
FDDI is a network standard which is often used for backbones. Optical waveguides (OWG), i.e., glass fibers, which provide optimum protection against electromagnetic interference, are generally used for transmission. More economical copper lines are also used for short transmission paths which provide the same transmission rate.
FDDI is an ANSI (American National Standards Institute) network standard whose network topology is designed in the shape of a ring. Most of the parameters are defined in ANSI X3T9.5, and parts have been adopted by ISO (International Organization for Standardization). The current version of the standard is defined in ANSI X3T12.
The FDDI standard supports multiple designs of the network topology, and the dual-ring structure is described below by way of example.
An FDDI network having a dual-ring structure includes a primary (p) ring and a secondary (s) ring. Each station has one input interface (E), i.e., an input pE, sE, and one output interface (A), i.e., an output pA, sA, for each ring. The primary and secondary rings have opposite directions of transmission.
During normal data transmission, each station forwards the data it has received at one input to the corresponding output. This takes place regardless of whether the data is intended for that station and is therefore also processed by this station. When the data is returned to the original sender, the data transmission has been completed correctly, and the original sender takes the data from the ring.
The secondary ring remains unused in normal, error-free operation. Nevertheless, null data is transmitted to continuously check whether this ring is free of errors.
If the dual ring is interrupted, e.g., due to a cable defect, the data transmitted by a station on the primary ring is not returned to that station.
If an error occurs, a ring interruption is detected or a time limit is exceeded, a claim process is initiated. If this process is unsuccessful, a beacon process is triggered.
Stations which do not receive any corresponding frames during the course of the beacon process either identify the preceding station or the fiber-optic cable as defective and initiate a ring reconfiguration.
To do this, stations which are located upstream from the cable rupture in the direction of the ring stop forwarding the data received via the pE port and instead reroute it to the secondary ring via the sA port. Because this ring uses the opposite direction of data flow, the rupture point is thereby bypassed.
A method for detecting a line interruption is discussed in International patent application WO 02/065219 A2, in which a master evaluates its own transmitted telegrams which are returned to the master via the dual ring. If the master's telegrams fail to be returned, this is evaluated as a line interruption.
The above-discussed method may require a great deal of time to detect and subsequently eliminate a fault, which may hinder its use in systems which require a nearly uninterrupted connection, such as industrial manufacturing systems.
There is also a method for quickly detecting a line break in dual-ring OWG structures via missing input signal edges and subsequently reconfiguring the two rings (“Fehlertolerantes Kommunikationssystem für hochdynamische Antriebsregelungen” [Error-Tolerant Communication Systems for Highly Dynamic Drive Regulation Systems], S. Schulze, Dissertation, Darmstadt, 1995). Glass fibers are highly sensitive in their handling and require complex connecting techniques. In addition, the use of OWG technology increases the cost of providing and maintaining the transmission medium itself as well as plugs, network cards, etc.